The importance of accurately and consistently controlling tension or preload applied to threaded fasteners increases with precision or criticality of parameters and tolerances of the assembly as a whole. This is particularly true in mass production of precision-designed equipment which may later be subjected to maintenance or repair, following which load applied to the assembly fasteners must be substantially the same as that applied during original manufacture. For example, in manufacture of internal combustion engines designed for high performance and fuel economy, the head is fastened to the engine block with a plurality of bolts prior to final machining of various block/cylinder critical surfaces. In the event that the head is later removed for repair or replacement, it is important that the same be precisely reassembled to the block so as to restore relationships of critical surfaces obtained during the original manufacturing machining operations.
Conventionally, preloading of threaded fasteners in engine and other assembly applications is controlled by monitoring torque applied to the assembly tool, such as with a mechanical or electrical torque wrench. Fastener preload control through monitoring of fastener torque alone, however, yields unpredictable and inconsistent results due in part to varying friction between the mating threads and beneath the fastener head. Where it has been attempted to obtain greater uniformity through use of lubricants or the like, results have continued to be unsatisfactory. Another approach has been to monitor torque as a function of angle of rotation, determine rate of change of torque, and compare the resulting data during the manufacturing operation to empirically determine data prestored in a computer memory. Such arrangements still do not directly measure fastener tension, and in addition require expensive assembly and control hardware. A third approach has been to tighten the fastener to a point at which the fastener material yields and the fastener head separates from the threaded body. Arrangements of this type suffer from the same inherent drawbacks as the torque wrench technique described above due to varying friction between the fastener and the assembly, and also increases the cost of both manufacture and repair due to requirement for special double-headed fasteners. For a general discussion of conventional techniques for monitoring and controlling pretensioning of threaded fasteners, see Kelly, "Electronic Controls Zero-In On Reliable Fastening," Assembly Engineering, November 1986, pages 34-38.
A further technique for controlling fastener preload has been found to yield particularly consistent results. This technique, termed "torque-turn" or "torque-angle," involves initially tightening the fastener to a specified torque, and thereafter tightening the fastener through an additional prespecified angle. The initial tightening torque is empirically predetermined to be one at which the fastener is tightened in assembly but has not yet been substantially elastically stretched. By thereafter tightening the fastener through an additional angle or fraction of a turn, advantage is taken of the precision machining of the fastener threads so as to obtain a predetermined elastic stretching of the fastener within the assembly. For example, a torque-turn or torque-angle fastening specification ma call for initial tightening to a torque of 25 Newton-meters, followed by an additional one-half turn or 180.degree. rotation in three equal steps. Computer-based equipment has been proposed for implementing such fastener preloading technique in mass production operations. However, as previously noted, control during maintenance and repair is as important as control during original assembly, and there remains a need in the art for inexpensive equipment which may be employed by maintenance and repair technicians in the field for obtaining the same precision control of fastener preloading as is done during the original manufacturing operation. A general object of the present invention is to provide apparatus of such a character.